Germanium oxidation states

M.A. Hanson, V.V. Terskikh, K.M. Baines, Y. Huang, Chlorine-35 solid-state NMR spectroscopy as an indirect probe of germanium oxidation state and coordination environment in germanium chlorides, Inorg. Chem. 53 (2014) 7377-7388. 

The oxidation state -1-4 is predominantly covalent and the stability of compounds with this oxidation state generally decreases with increasing atomic size (Figure 8.1). It is the most stable oxidation state for silicon, germanium and tin, but for lead the oxidation state +4 is found to be less stable than oxidation state +2 and hence lead(IV) compounds have oxidising properties (for example, see p. 194). 

The concept of oxidation states is best applied only to germanium, tin and lead, for the chemistry of carbon and silicon is almost wholly defined in terms of covalency with the carbon and silicon atoms sharing all their four outer quantum level electrons. These are often tetrahedrally arranged around the central atom. There are compounds of carbon in which the valency appears to be less than... 

Silicon, germanium, tin and lead can make use of unfilled d orbitals to expand their covalency beyond four and each of these elements is able (but only with a few ligands) to increase its covalency to six. Hence silicon in oxidation state -f-4 forms the octahedral hexafluorosilicate complex ion [SiFg] (but not [SiCl] ). Tin and lead in oxidation state -1-4 form the hexahydroxo complex ions, hexahydroxostannate(IV). [Sn(OH) ] and hexahydroxoplum-bate(IV) respectively when excess alkali is added to an aqueous solution containing hydrated tin(IV) and lead(IV) ions. 

The concentration of Ge in sea water and in the earth s crust have been estimated at 0.05 Agl-1 and 2 mg kg-1, respectively9. Germanium is generally found in the +4 oxidation state as the oxide or in solution as germanic acid under various environmental conditions. 

Germanium monoxide and germanium dioxide are examples of the elements oxidation states of +2 and +4, as follows ... 

The stability of the germanium and silicon porphyrin alkylperoxide complexes contrasts with the lability of the corresponding iron porphyrin alkylperoxide complexes. For example, Fe(Por)(OOEt) was formed from the reaction of Fe(Por)Et with O2 and has been characterized only by spectroscopy as it decomposes to Fe(Por)OH and CH3CHO above —80°C. The key to the difference in stability is likely to be the availability of higher oxidation states in the iron porphyrins as... 

Por )=0 intermediates. Many metalloporphyrin complexes are bleached by peroxides, but the germanium porphyrins remain intact even in the presence of excess alkylhydroperoxide. This suggests that high oxidation state intermediates... 

The chemical properties of germanium fall between those of silicon and tin. It forms both the divalent and tetravalent compounds, the oxidation state +4 being more stable than the +2 oxidation state. The metal is stable in air and water at ambient temperatures. However, it reacts with oxygen at elevated temperatures forming divalent and tetravalent oxides, GeO and Ge02. 

J. F. Surface oxidation states of germanium. Surface Science 172, 455-65 (1986). 

Conversely, the stability of compounds of oxidation state +11 increases dramatically as the atomic number of the element increases. Carbenes, CX2, and silylenes, SiX2, are well established as transient reaction intermediates, and structural data have been obtained in several cases either at high temperatures or by their generation in low-temperature matrices. However, only for germanium, tin and lead are compounds in this oxidation state stable under ordinary conditions. Compounds with the Group IV element in oxidation state +111, the formal oxidation state of the radical species, R3M, are also usually considered as unstable transients. However, when R is very bulky, these metal-centred radicals, such as for example Sn[CH(SiMe3)2]3, have extremely long, perhaps indefinite, lifetimes in solution. 

Zinc(ll). gallium(lll), and germanium(IV) are the most stable oxidation states for these elements, but the later nonmetals (arsenic, selenium, and bromine) show a reluctance to assume their highest possible oxidation state. 

Before seeking an explanation of the reluctance of As, Se, and Br to exhibit maximum oxidation states, a related phenomenon will be explored. This involves a tendency for germanium to resemble carbon more than silicon. Some examples arc ... 

It is apparent from most of the examples previously described that the most common formal oxidation state found for the Group 14 element is E(IV) (E = Ge, Sn, Pb). Relatively few examples of divalent germanium, tin, or lead complexes have been described, and of these, many are not well characterized. Cobalt-containing compounds are no exception in this regard and there appears to be only one report in the literature that describes a species of this type, viz. [Ge Co(CO)4 2], 67, although the precise structure of this complex is unknown (77). Two main synthetic routes are described, Eqs. (4) and (S), the starting complex in the latter reaction being... 

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